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<div class="titlepage"><div><div><h5 class="title">
<a name="spirit.qi.reference.auxiliary.attr"></a><a class="link" href="attr.html" title="Attribute Parser (attr)">Attribute Parser
          (<code class="computeroutput"><span class="identifier">attr</span></code>)</a>
</h5></div></div></div>
<h6>
<a name="spirit.qi.reference.auxiliary.attr.h0"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.attr.description"></a></span><a class="link" href="attr.html#spirit.qi.reference.auxiliary.attr.description">Description</a>
          </h6>
<p>
            The Attribute parser does not consume any input, for this reason it always
            matches an empty string and always succeeds. It's purpose is to expose
            its specified parameter as an attribute.
          </p>
<h6>
<a name="spirit.qi.reference.auxiliary.attr.h1"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.attr.header"></a></span><a class="link" href="attr.html#spirit.qi.reference.auxiliary.attr.header">Header</a>
          </h6>
<pre class="programlisting"><span class="comment">// forwards to &lt;boost/spirit/home/qi/auxiliary/attr.hpp&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi_attr</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
            Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>.
          </p>
<h6>
<a name="spirit.qi.reference.auxiliary.attr.h2"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.attr.namespace"></a></span><a class="link" href="attr.html#spirit.qi.reference.auxiliary.attr.namespace">Namespace</a>
          </h6>
<div class="informaltable"><table class="table">
<colgroup><col></colgroup>
<thead><tr><th>
                    <p>
                      Name
                    </p>
                  </th></tr></thead>
<tbody><tr><td>
                    <p>
                      <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">attr</span> <span class="comment">// alias:
                      boost::spirit::qi::attr</span></code>
                    </p>
                  </td></tr></tbody>
</table></div>
<h6>
<a name="spirit.qi.reference.auxiliary.attr.h3"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.attr.model_of"></a></span><a class="link" href="attr.html#spirit.qi.reference.auxiliary.attr.model_of">Model
            of</a>
          </h6>
<div class="blockquote"><blockquote class="blockquote"><p>
              <a class="link" href="../parser_concepts/primitiveparser.html" title="PrimitiveParser"><code class="computeroutput"><span class="identifier">PrimitiveParser</span></code></a>
            </p></blockquote></div>
<div class="variablelist">
<p class="title"><b>Notation</b></p>
<dl class="variablelist">
<dt><span class="term"><code class="computeroutput"><span class="identifier">a</span></code></span></dt>
<dd><p>
                  A arbitrary typed constant value, e.g. 0.0, "Hello",
                  or a variable of arbitrary type or a <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy
                  Argument</a> that evaluates to an arbitrary type.
                </p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">A</span></code></span></dt>
<dd><p>
                  The type of <code class="computeroutput"><span class="identifier">a</span></code> or
                  if it is a <a class="link" href="../basics.html#spirit.qi.reference.basics.lazy_argument">Lazy
                  Argument</a>, its return type.
                </p></dd>
</dl>
</div>
<h6>
<a name="spirit.qi.reference.auxiliary.attr.h4"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.attr.expression_semantics"></a></span><a class="link" href="attr.html#spirit.qi.reference.auxiliary.attr.expression_semantics">Expression
            Semantics</a>
          </h6>
<p>
            Semantics of an expression is defined only where it differs from, or
            is not defined in <a class="link" href="../parser_concepts/primitiveparser.html" title="PrimitiveParser"><code class="computeroutput"><span class="identifier">PrimitiveParser</span></code></a>.
          </p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Expression
                    </p>
                  </th>
<th>
                    <p>
                      Semantics
                    </p>
                  </th>
</tr></thead>
<tbody><tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">attr</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      Create a pseudo parser exposing the current value of <code class="computeroutput"><span class="identifier">a</span></code> as its attribute without
                      consuming any input at parse time.
                    </p>
                  </td>
</tr></tbody>
</table></div>
<h6>
<a name="spirit.qi.reference.auxiliary.attr.h5"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.attr.attributes"></a></span><a class="link" href="attr.html#spirit.qi.reference.auxiliary.attr.attributes">Attributes</a>
          </h6>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                    <p>
                      Expression
                    </p>
                  </th>
<th>
                    <p>
                      Attribute
                    </p>
                  </th>
</tr></thead>
<tbody><tr>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">attr</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span></code>
                    </p>
                  </td>
<td>
                    <p>
                      <code class="computeroutput"><span class="identifier">A</span></code>
                    </p>
                  </td>
</tr></tbody>
</table></div>
<h6>
<a name="spirit.qi.reference.auxiliary.attr.h6"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.attr.complexity"></a></span><a class="link" href="attr.html#spirit.qi.reference.auxiliary.attr.complexity">Complexity</a>
          </h6>
<div class="blockquote"><blockquote class="blockquote"><p>
              O(1)
            </p></blockquote></div>
<p>
            The complexity is constant as no input is consumed and no matching is
            done.
          </p>
<h6>
<a name="spirit.qi.reference.auxiliary.attr.h7"></a>
            <span class="phrase"><a name="spirit.qi.reference.auxiliary.attr.example"></a></span><a class="link" href="attr.html#spirit.qi.reference.auxiliary.attr.example">Example</a>
          </h6>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
              The test harness for the example(s) below is presented in the <a class="link" href="../basics.html#spirit.qi.reference.basics.examples">Basics Examples</a>
              section.
            </p></td></tr>
</table></div>
<p>
            Some using declarations:
          </p>
<p>
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">phx</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">attr</span><span class="special">;</span>
</pre>
<p>
          </p>
<p>
            Using <code class="computeroutput"><span class="identifier">attr</span></code> with literals:
          </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">str</span><span class="special">;</span>
<span class="identifier">test_parser_attr</span><span class="special">(</span><span class="string">""</span><span class="special">,</span> <span class="identifier">attr</span><span class="special">(</span><span class="string">"boost"</span><span class="special">),</span> <span class="identifier">str</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">str</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>            <span class="comment">// will print 'boost'</span>

<span class="keyword">double</span> <span class="identifier">d</span><span class="special">;</span>
<span class="identifier">test_parser_attr</span><span class="special">(</span><span class="string">""</span><span class="special">,</span> <span class="identifier">attr</span><span class="special">(</span><span class="number">1.0</span><span class="special">),</span> <span class="identifier">d</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">d</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>              <span class="comment">// will print '1.0'</span>
</pre>
<p>
          </p>
<p>
            Using <code class="computeroutput"><span class="identifier">attr</span></code> with <a href="../../../../../../../../libs/phoenix/doc/html/index.html" target="_top">Boost.Phoenix</a> function
            objects:
          </p>
<p>
</p>
<pre class="programlisting"><span class="identifier">d</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">;</span>
<span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">1.2</span><span class="special">;</span>
<span class="identifier">test_parser_attr</span><span class="special">(</span><span class="string">""</span><span class="special">,</span> <span class="identifier">attr</span><span class="special">(</span><span class="identifier">phx</span><span class="special">::</span><span class="identifier">ref</span><span class="special">(</span><span class="identifier">d1</span><span class="special">)),</span> <span class="identifier">d</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">d</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>              <span class="comment">// will print '1.2'</span>
</pre>
<p>
          </p>
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<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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